Al-Ce-Zr Nanocomposites Supported Pd Catalysts For Selective Catalytic Reduction Of NO_x With Hydrogen

Nitrogen oxides(NOx)emissions from vehicles and stationary sources have been strictly regulated because of their nagtive impact on the environment and human health.Hydrogen selective catalytic reduction(H2-SCR)technology has attracted extensive research interest since the technique has excellent catalytic activity at low operation temperature(<200 ℃).In this thesis,we synthetized Al-Ce-Zr nanocomposite oxides supports by coprecipitation method,and a series of Pd catalysts were provided by wet incipient impregnation method.All catalysts were employed in H2-SCR deNOx process and the performance were evaluated by NO,cconversion and N2 selecticvity.Also,XRD,BET,TEM,HRTEM,Element Mapping,H2 pulse adsorption,H2 and CO adsorption isotherm,TPD and XPS characterizations were carried out to analyze the physical and chemical properties of the catalysts.The influence of Pd mass loading(0～1 wt%)on deNOx performance was studied by comparing the catalytic activity of 1%Pd/Al-CeZrO4,0.75%Pd/Al-CeZrO4,0.5%Pd/Al-CeZrO4,0.25%Pd/Al-CeZrO4 catalysts and Al-CeZrO4 as control at Gas Hourly Space Velocity(GHSV)= 60,000 h-1 and GHSV = 24,000 h’1.The results indicated that the catalytic activity of the catalysts was improved with the increase of Pd loading.Among them,the NO.conversion on 1%Pd/Al-CeZrO4 was more than 80%in the temperature range of 110～300 ℃ at the GHSV = 60,000 h-1)and was higher than those of other catalysts with less Pd loading.The effects of CeZrO4 and Al-CeZrO4 supports on the catalytic activity of Pd/CeZrO4 and Pd/Al-CeZrO4 were investigated in the temperature range of 100～300 ℃.The Pd/CeZrO4 catalyst showed "bimodal-type"behavior of catalytic activity in the temperature range of 100～300 ℃,while Pd/Al-CeZrO4 catalyst exhibited high catalytic activity(NOx conversion over 85%)at 100～300 ℃ range,which indates a greatly broadened operating temperature range of the catalyst.The introduction of Ni showed improved NOx conversion and N2 selectivty at the same time.Particularly,the N2 selectivty of the Pd-Ni/Al-CeZrO4 catalyst was beyond 80%at 80～200 ℃.XRD results showed that the supports were consistent with the cubic CeZrO4,and the crystallite size of CeZrO4 is 4.1 nm.The introduction of Al did not change the crystal phase of the cubic CeZr04,but the crystallite size was reduced to 3 nm.N2 isothermal adsorption experiments showed that a specific surface area of 109 m2g-1 for the CeZrO4 while 143 m2g-1 for the Al-CeZrO4,which means the introduction of Al brought changes in textural properties of the oxide.XRD patterns and TEM images did not show the presence of Pd particles,but Element Mappings displayed that Pd were highly dispersed and plenty of spots with higher contrast were found,which were ascribable to Pd nanoclusters,in the HRTEM images.Further,from the XPS spectrum of elemental Pd,it was found that the Pd nanoclusters in the catalysts were PdO and PdOx species.In addition,the active specific surface area of metal Pd of Pd/Al-CeZrO4 catalyst(115.3 m2g-1)was about twice that of Pd/CeZrO4(68.9 m2g-1)according to the results of H2 pulsed chemisorption.And the Pd dispersion and hydrogen spillover of Pd/Al-CeZrO4 catalysts(76.6%and 207.7%)were higher than those of Pd/CeZrO4 catalysts(39.3%and 104.1%)calculated by CO and H2 isothermal adsorption.The results of H2-TPD characterization showed that the Al-addition catalysts had a higher initiative desorption temperature(200 ℃)than that of the catalyst without Al-addition(60 ℃),indicating a stronger H2 adsorption ability of the Pd/Al-CeZrO4 catalyst.NO-TPD results showed that Pd/Al-CeZrO4 and Pd-Ni/Al-CeZrO4 catalysts had an ability to adsorb more NO on the surface than Pd/CeZrO4,and similar conclusions can be drawn from the XPS spectra and calculations of the surface Oadsorbed and Ce3+contents.The Oadsorbea content were 42.6%,69.0%and 70.3%,while the Ce3+ contents were 15.4%,25.0%and 26.7%for Pd/CeZrO4,Pd/Al-CeZrO4 and Pd-Ni/Al-CeZrO4,respectively.The results demonstrated that Pd/Al-CeZrO4 and Pd-Ni/Al-CeZrO4 catalysts are more likely to adsorb oxygen and NO.From the TPRS results,we could make a reasonable assumption of the H2-SCR reaction path at catalysts’ surface.In summary,by consolidating the results of various characterization and activity tests,it is reasonable to believed that the Pd-based catalysts we designed and synthetized in this thesis showed excellent catalytic performance in H2-SCR process,which greatly broaden the operating temperature window of the catalysts and have a great potential for industrial application.